A Multiple Input Multiple Output (MIMO) technology will hereinafter be described in detail. In brief, the MIMO technology is an abbreviation of the Multi-Input Multi-Output technology. The MIMO technology uses multiple transmission (Tx) antennas and multiple reception (Rx) antennas to improve the efficiency of Tx/Rx data, whereas a conventional art has generally used a single transmission (Tx) antenna and a single reception (Rx) antenna. In other words, the MIMO technology allows a transmitting end or receiving end of a wireless communication system to use multiple antennas (hereinafter referred to as a multi-antenna), so that the capacity or performance can be improved. For the convenience of description, the term “MIMO” can also be considered to be a multi-antenna technology.
In more detail, the MIMO technology is not dependent on a single antenna path to receive a single total message, collects a plurality of data pieces received via several antennas, and completes total data. As a result, the MIMO technology can increase a data transfer rate within a specific coverage, or can increase system coverage at a specific transfer rate. In other words, the MIMO technology is the next-generation mobile communication technology capable of being applied to mobile communication terminals or relays.
The MIMO technology from among a variety of technologies can greatly increase an amount of communication capacity and Tx/Rx performances without allocating additional frequencies or increasing an additional power. Due to these technical advantages, most companies or developers are intensively paying attention to this MIMO technology. The next-generation mobile communication technology requires a data transfer rate higher than that of a conventional mobile communication technology, so that it is expected that the effective MIMO technology is requisite for the next-generation mobile communication technology. Under this situation, the MIMO communication technology is the next-generation mobile communication technology capable of being applied to mobile communication terminals or relays, and can extend the range of a data communication range, so that it can overcome the limited amount of transfer data of other mobile communication systems due to a variety of limited situations.
The above-mentioned MIMO technology can be classified into a spatial diversity scheme and a spatial multiplexing scheme. The spatial diversity scheme increases transmission reliability using symbols passing various channel paths. The spatial multiplexing scheme simultaneously transmits a plurality of data symbols via a plurality of Tx antennas, so that it increases a transfer rate of data. Detailed descriptions of the spatial diversity scheme, the spatial multiplexing scheme, and the combination thereof will hereinafter be described in detail.
Firstly, the spatial diversity scheme will hereinafter be described. The spatial diversity scheme is classified into a space-time block code scheme and a space-time Trellis code scheme which simultaneously uses a diversity gain and a coding gain. Generally, a bit error ratio (BER) improvement performance and a code-generation degree of freedom of the space-time Trellis code scheme are superior to those of the space-time block code scheme, whereas the calculation complexity of the space-time block code scheme is superior to that of the space-time Trellis code scheme. The above-mentioned spatial diversity gain corresponds to the product or multiplication of the number (NT) of Tx antennas and the number (NR) of Rx antennas, as denoted by NT×NR.
Secondly, the spatial multiplexing scheme will hereinafter be described. The spatial multiplexing scheme is adapted to transmit different data streams via individual Tx antennas. In this case, a receiver may unavoidably generate mutual interference between data pieces simultaneously transmitted from a transmitter. The receiver removes this mutual interference from the received data using a proper signal processing technique, so that it can receive the resultant data having no interference. In order to remove noise or interference from the received data, a maximum likelihood receiver, a ZF receiver, a MMSE receiver, a D-BLAST, or a V-BLAST may be used. Specifically, if a transmitting end can recognize channel information, a Singular Value Decomposition (SVD) scheme may be used to remove the noise or interference.
Thirdly, the combination of the spatial diversity scheme and the spatial multiplexing scheme will hereinafter be described. Provided that only a spatial diversity gain is acquired, the performance-improvement gain is gradually saturated in proportion to an increasing diversity order. Otherwise, provided that only the spatial multiplexing gain is acquired, a transmission reliability of a RF channel is gradually deteriorated. As a result, a variety of schemes capable of acquiring all the above-mentioned two gains simultaneously while solving the above-mentioned problems have been intensively researched by many companies or developers, for example, a double-STTD scheme and a space-time BICM (STBICM) scheme.